1. Field of the Invention
The invention generally relates to the measurement of rotational speed and is more particularly concerned with a simple unitary eddy current proximity sensing device cooperating with an electrically conductive rotor having a specially perturbated peripheral shape for providing shaft speed and direction of rotation in the form of electrical output signals.
2. Description of the Prior Art
While there are several choices available among prior art tachometers which supply an output voltage generally related to shaft speed, such devices are often not suitable for certain precision applications, such as in control moment gyroscopes or reaction wheel assemblies for use in communication or other space satellites of the class, for example, illustrated in the L. P. Davis U.S. Pat. No. 3,980,358 for "Axial Vibration Damper for Floating Bearings", issued September, 1976 and assigned to Sperry Rand Corporation, and in the art cited therein. Many prior art tachometer devices are characterized by excessive or variable friction effects that cannot be tolerated where such energy losses must be fully avoided. Additional disadvantages include a ripple voltage on the output signal, life shortening wear, and decreasing reliability with age. Alternating current tachometers exchange some of these defects for relatively poor linearity and a finite displacement error at zero speed; the output frequency varies among with the magnitude of the output signal.
Brushes and commutators or slip rings are avoided in other inductive types of shaft speed measurement devices known in the art in which no physical contact is made to rotatable parts. In one such device, at least one small magnet is disposed at the periphery of a rotor and induces signals in a probing coil located near the disc edge. However, the probe coil output amplitude is an inherent function of shaft speed, which severely limits the useful measurement range of the device, making it useless at low shaft speeds. In some cases, the magnets undesirably add inertia to particular kinds of rotating systems. Dynamic magnetic fields are also produced that may not be compatible with the proper operation of associated apparatus.
Commutated condenser tachometric devices are available which avoid friction, advantageously lacking direct mechanical-electrical contacts, in which a capacitor is repeatedly charged to one potential and is discharged to another through a current-measuring device, the consequent output pulses comprising a current whose unidirectional component is proportional to shaft speed. The output is non-linear and is erratic at low shaft speeds and, as in the instance of other prior art concepts, there is no simple or reliable indication of the direction of shaft rotation.